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Electrostatics

Electrostatics. Learning Objectives. The electrostatic force (Coulomb’s Law) can be either repulsive or attractive (SOL 12.a)

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Electrostatics

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  1. Electrostatics

  2. Learning Objectives • The electrostatic force (Coulomb’s Law) can be either repulsive or attractive (SOL 12.a) • The interaction of two particles can be described as: the creation of a field by one of the particles and the interaction of the field with the second particle (SOL 12.b).

  3. Magnitude of charge on protons and electrons are exactly the same • Protons have a positive charge • Electrons have a negative charge • Neutral atoms contain equal numbers of protons and electrons

  4. Insulators and ConductorsNeed to know • Insulator: electrons are bound very tightly to the nuclei. Wood and rubber are good insulators. • Conductor: electrons are bound very loosely and can move about freely. They are often referred to free electrons. Metals are good conductors. • Semiconductor: very few free electrons (silicon, germanium and carbon)

  5. Static Electricity • You have probably experienced a charge lately (comb, dryer, carpet, car seat, …) • An object becomes charged due to a rubbing process and is said to possess a net electric charge • An item containing a net positive charge has lost electrons • An item containing a net negative charge has gained electrons

  6. Law of Conservation of Electric ChargeNeed to know The net amount of electric charge produced in any process is zero If one object or one region of space acquires a positive charge, then an equal amount of negative charge will be found in neighboring areas or objects

  7. Unlike Charges Attract;Like Charges RepelNeed to know

  8. 3 Ways to Charge an ObjectNeed to know • Friction: Rubbing two objects together with different electron attachment. Heat generated frees electrons to join object with stronger attachment.

  9. 3 Ways to Charge an ObjectNeed to know 2. Conduction: Electrons are transferred from one object to another by touching. Usually it involves moving from one electric potential to another. John TraVOLTa Demo

  10. 3 Ways to Charge an ObjectNeed to know 3. Induction: Rod does not touch sphere. It pushes electrons out of the back side of the sphere and down the wire to ground. The ground wire is disconnected to prevent the return of the electrons from ground, then the rod is removed.

  11. Electromagnetism • One of the four fundamental forces of the universe (electromagnetism, gravity, weak nuclear and strong nuclear forces) • The forces that act between atoms and molecules to hold them together are electrical forces • Elastic, normal and contact forces (pushes and pulls) result from electric forces acting at the atomic level

  12. Forces resulting from charges • Charges push and pull on one another • Closer the charge the higher the force • The stronger the charge the higher the force

  13. Coulomb’s LawNeed to know The magnitude of the force between charge qA and charge qB, separated a distance d, is proportional to the magnitude of the charge and inversely proportional to the square of the distance: F = K qAqB d2 qA qB d

  14. Coulomb’s Law: Key FactsNeed to know The charge of an electron is: -1.60 x 10-19 coulombs (C) The charge of a proton is: 1.60 x 10-19 coulombs (C) The charge, q, is measured in coulombs. The distance, d, is measured in meters. The force, F, is measured in newtons. The constant, K = 9.0 x 109 Nm2/C2

  15. Problem Solving Strategy • Sketch the system showing all distances • Diagram the vectors • Use Coulomb’s law to find the magnitude of the force. Note: it is unnecessary to include the sign of the charges or the distance. The answer is always positive. • Use your diagram along with trigonometric relations to find the direction of the force

  16. Example Problem 1 Two charges are separated by 3.0 cm. Object A has a charge of +6.0 C, while object B has a charge of +3.0 C. What is the force on object A? Known: Unknown: qA = +6.0 x 10-6 C FB on A = ? qB = +3.0 x 10-6 C d = 0.030 m

  17. Example 1 Solution F = K qAqB d2 = (9.0 x 109 Nm2/C2)(6.0 x 10-6C)(3.0 x 10-6C) (3.0 x 10-2 m)2 FB on A = 1.8 x 102 N

  18. +6 µC -2 µC 2 µC 2 cm 6 cm Example 2: Three Charges • Given: • Find the net force on the -2 µC charge • Known:

  19. +6 µC -2 µC 2 µC 2 cm 6 cm FA on B= KqAqB d2 = (9x109Nm2C2)(6x10-6 C)(2x10-6 C) (0.06 m)2 = - 30 N FC on B= KqCqB d2 = (9x109Nm2C2)(2x10-6 C)(2x10-6 C) (0.02 m)2 = + 90 FNet = FA on B + FC on B = - 30 N + 90 N = 60 N

  20. Fnet FC on A B A  dAB FB on A qB = -3 C qA= 6 C dAC C qc = 1.5 C Example Problem 3 A sphere with a charge 6.0 C is located near two other charged spheres. A -3.0 C is located 4.00 cm to the right and a 1.5 C sphere is located 3.00 cm directly underneath. Determine the net force on the 6.0 C sphere.

  21. Fnet FC on A B A  dAB FB on A qB = -3 C qA= 6 C dAC C qc = 1.5 C Example 3 Solution FB on A = FC on A = Fnet =  =

  22. Static Charge Generator

  23. Electric FieldNeed to know • An electric field extends outward from every charge and permeates all of space

  24. +qB qA+ Investigating the Electric Field • We can quantify the strength of an electric field by measuring the force on a small positive test charge • So small that the force it exerts does not significantly alter the distribution of the charges that create the field a

  25. +qB qA+ Electric Field • An electric field, E, at any point is defined as the force, F, exerted on a tiny positive test charge at that point divided by the magnitude of the test charge: E = F/qB

  26. +qB qA+ Electric Field Equation E = F/qB E = K qB qA/r2 qB E = KqA/r2

  27. +qB qA+ Electric Field Lines • Drawn so that they indicate the direction of the force due to the given field on a positive charge

  28. Electric Field LinesNeed to KnowLines indicate direction of the force due to the given field on a positive test charge

  29. Properties of Field LinesNeed to know • The field lines indicate the direction of the electric field • The lines are drawn so that the magnitude of the electric field, E, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer the lines, the stronger the field. • Electric field lines start on positive charges and end on negative charges

  30. Electric Potential DifferenceNeed to know V = Won q’ = PE: Potential difference often q’ q’ referred to as Voltage Electric Potential Difference Units: Volt =J/C + g E displacement displacement W = Fd = mgd W = Vq + Big Negative Charge

  31. Source Thundercloud to ground High voltage power line Power supply for TV tube Auto ignition Household outlet Auto battery Resting potential across nerve membrane Potential changes on skin (EKG) Voltage 108 V 106 V 104 V 104 V 102 V 12 V 10-1 V 10-4 V Typical Voltages

  32. CapacitorsNeed to Know • A capacitor is a device that can store electric charge • Consists of two conducting objects placed near each other but not touching • They store charge for later use • Usage: camera flash, energy back-up for computers and as surge protectors

  33. Capacitors • Consists of a pair of parallel plates of area, A, and separated by a small distance d. • In a diagram, they are represented by the symbol: • If a voltage is applied to a capacitor, one plate acquires a negative charge and the other an equal amount of positive charge.

  34. CapacitorsNeed to Know • The amount of charge acquired by each plate is proportional to the potential difference Q = CV • Where C is constant and is called the capacitance of the capacitor • Unit: Coulombs/Volt = Farad • Typical capacitor range is 1pF (10-12) to 1F (10-6)

  35. Determining Capacitance • Constant for a given a capacitor • Depends on structure and dimensions of he capacitor itself: C = o A/d A = area d = separation distance between plates o = 8.85 x 10-12 C2/Nm2 = permittivity of free space

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